I agree it adds some drag but I wouldn't call it needless. If you are going to heat the cabin with a system that uses heat muffs, you have to keep the muffs cool.
If you use something to direct the waste cooling air outboard you are controlling where it goes but not likely having much influence on the drag that is produced.

Yeah. Darn shame eh?

Empirical data from multiple installations, including mine, have shown that this is not always the case. My aircraft uses a slide valve from Tom Berge, as does Tom's aircraft as do quite a few other aircraft I know of. Almost 900hrs and no issues to date on mine, and others have experienced similar reliability.

A simple deflector will work. Mine was discharging directly on the accumulator, not a good idea.

Yes I think you are worried about airflow that just isn't going to happen. Normally the heated air travels through your firewall valve from the the engine compartment in to the cockpit since the engine area (firewall forward) is at a higher pressure than the cockpit. Air always moves from high to low pressure regions.

Since that valve just opens the engine area in to the same engine area, there won't be airflow from the heat muff to the valve. The two areas are at approximately the same pressure.

Andrew Anunson

I think the danger with this type of information is that there is no specifics provided.
We can all agree that the surface temp of the pipe adjacent to the cyl head is quite different than what it is where the pipe exits the cowl.
So, depending on where a muff is located along the length of the pipe could have a major effect on the actual temp when the air is static in the system.

DanH's might with the closed cowl flap, but others? (yes, I know you don't have one) Are you sure for the lower cowl, data? Sounds like an exhaust contamination problem.

Yes it is.

I chalk it up as one of the engineering trade-offs we make balancing benefit against simplicity.

Similar to the decision of putting retractable gear on an RV. The performance benefit would be small so the cost and complexity is not worth it.

But I am open to ideas for a great performing heat source that is guaranteed to work for any installation, but requires no constant system flow.

Not true Andrew

The traditional system has the heater air sourced from the high pressure side (top of engine) of the cooling plenum.
Air moves through the system when the valve is in the off position, they same as it does through the engine cyl.

Scott,

While some constant heat muff airflow is nice to prolong the life of the attached SCAT hose, I do not believe the heat muff itself is going to melt without cooling air. The heat muffs I use are made of Stainless Steel, just like the exhaust pipes. For that matter since I believe no one mounts heat muff at the cylinder exhaust port, the exhaust pipe temperature at the heat muff is much lower than at the exhaust port.

So as all things go, there are extremes that bound the problem. No heatmuff flow is probable not good for the SCAT hose. Too much heatmuff flow is a waste of engine cooling air - and for that matter not a great way to heat the cabin. Heat to the cabin is a function of pounds mass of air and temperature. The more airflow, the less temperature rise across the heat muff. Those interested students out there can take the data then do the equations to find the airflow that transfers the most heat to the cabin.

As I previously posted, a 3/4? orifice to reduce the heat muff airflow in the RV-10 still provides far more cabin heat that I?ll ever use. For the RV-8A single heat muff install, I increased the heat transfer area in the muff to get the added cabin heat - as well as reduce the airflow though the muff (less pounds mass of air but higher temp rise across the muff). 900+ hours in the RV-8A install and all is well. I?ll do the same for the new RV-8 project.

Carl